The FTP function in eXtplorer uses vsftpd to provide web based (S?)FTP access. FTP uses individual Linux user accounts and groups so you can set individual and group rights via Linux user and group file/folder permissions.

The main eXtplorer interface uses a completely different user database all of it's own which doesn't allow the use of indivdual Linux user file/folder permissions - it all runs under the Linux webserver (www-data) user account. So if you wanted all users to have only read/download access you could use Linux file permissions for the folder (and all subfolders) of read only access to the www-data user account, that would mean that no users could write to the server though. There may be another way to acheive what you want using eXtplorer's own built in users, but I am unaware of it. If you'd like to investigate that path further then I suggest you contact the eXtplorer devs and/or community (after having a good google).